The Chernobyl Disaster

Nuclear power had become routine and have a bright atomic future ahead.

this nuclear power also pose a great threat on human civilization .

The Chernobyl disaster was one such nuclear accident that occurred on 26 April 1986, caused because of little misculation from the staff.

The Soviet Union had five power stations of this type but there is only one about which we all know The Chernobyl.

Chernobyl, also known as Chornobyl (Ukrainian: Чорнобиль) is a partially abandoned city in the Chernobyl Exclusion Zone, situated in the Vyshhorod Raion of northern Kyiv Oblast, Ukraine.

On Saturday 26 April 1986 there was something bad going to happen in the city so bad that even today we can see the trace of that.

On that weekend day in evening there was no hint of the catastrophe about to unfold that would have a profound impact on the USSR and nuclear power around the world.

At 1:23 am during a safety test the AZ-5 button was pushed. About 8 seconds later reactor 4 exploded.

The question which is asked today by almost everyone who knew a little about Chernobyl is that:-

Why did the emergency shutdown button (AZ -5) blow up the reactor?

The sole reason of their creation was to protect from any mishaps.

Let’s try to understand by looking at some facts,

The central piece of this power station was the RBMK reactor

It was a graphite moderated pressure tube reactor, using slightly enriched (2% U-235) uranium dioxide fuel.

(Graphite used to slow down very fast neutrons)

(Control rods made out of boron carbide to absorb very fast neutrons)

(Water absorbs the neutron, hence it acts as coolant and also creates electricity by turning the turbines)

The most important characteristics of the RBMK reactor is that it can possess a “positive void coefficient” which means presence of steam instead of water near the active rods , where an increase in steam bubbles (‘voids’) is accompanied by an increase in core reactivity i.e.

As steam production in the fuel channels increases, the neutrons that would have been absorbed by the denser water now produce increased fission in the fuel.

This Chernobyl’s reactor number 4 was only 2.5 years old , so the explosion was not because of the usage old instruments , instead the explosion was a result of “end effect” which was a design error in the reactor.

So what happened was that:-

On 25th April, the crew started to prepare for a test before a routine shutdown.

The test was to determine that how long turbine can spin to provide power to main circulating pumps after a cutoff of main electrical power supply.

The last 3 time they did this test the turbine ran off to quickly so now with new voltage regulator they were hoping for a success of the test.
The test had to be performed during a complete reactor shutdown. But on this day the shutdown was paused at 50 % power for most of the day the reason for that is because the electrical grid needed additional power and Chernobyl unit 4 had to make up the shortfall so after a 19 hour pause the shutdown was permitted to continue. This was a crucial contributors to the disaster.

As a result the reactor became “XENON POISONED”.

The test was performed by the less experienced night shift staff who had not been properly briefed about the ongoing test .

Then the new shift took over at midnight and put the reactor onto auto control to keep it steady at 22% power.

Except auto mode was not designed for a poisoned core Soon the power plummeted to 1% before the test had been done.

Xenon is a powerful neutron poison.

The worker had not noticed that because of 50 % shutdown xenon poisoning took place and made the reactor unstable. And the unstable nuclear reaction lead to disasters.

So

Xenon forms 6 hours later and interfered with the fine equilibrium of the nuclear reaction.

The operators noticed the power surge and realized they needed to reduce reactivity quickly by inserting more control rods. They hit the red button of the emergency power-reduction system. Motors began driving all 205 control rods as well as the emergency protection rods into the reactor core.

But the control rods had a design flaw that now proved deadly: their tips were made of graphite. as a result instead of slowing down the reaction the graphite tips increased it.

The AZ-5 button inserted all the top control rods simultaneously, but the lower control rods were not coupled with the button hence they did not moved.

I.e. only the rods for top came down but the bottom one did not moved up.

This peculiarity of the design to press AZ-5 button was done to shut down the reactor in a hurry if they ever need to.

So coming back to the topic

The interaction of very hot fuel with the cooling water (acting both as cooler and absorber of neutrons) led to fuel fragmentation along with rapid steam production and an increase in pressure. The over pressure caused the two million pound of cover plate of the reactor to dislodge, rupturing the fuel channels and jamming all the control rods, which by that time were only halfway down.

Causing a steam explosion and releasing fission products to the atmosphere. About two to three seconds later, a second explosion threw out eight tons of radioactive material into the atmosphere from the fuel channels and set the graphite on fire.
Hence, this was more of a of a heat and pressure explosion rather than a nuclear blast.

The accident caused the largest uncontrolled radioactive release into the environment ever recorded for any civilian operation, and large quantities of radioactive substances were released into the air for about 10 days the cloud of contamination covered over 200,000 square kilometres of land mass i.e. 1% of arable surface of earth.

The next task was cleaning up the radioactivity at the site so that the remaining three reactors could be restarted; hence

A. The damaged reactor was shielded permanently by covering with the two sarcophagus shaped containment facility over the reactor.

B. The Exclusion Zone was created which covers an area of approximately 2,800 Sq kilometre surrounding the Chernobyl Nuclear Power Plant where radioactive contamination is highest and public access and in habitation are restricted till next 6 centuries.

The Nature is making a comeback in old remains of the Chernobyl city.

Researchers have found the land surrounding the plant, which has been largely off limits to humans for three decades, has become a haven for wildlife, with lynx, bison, deer and other animals roaming through thick forests. This so-called Chernobyl Exclusion Zone (CEZ), which covers 2,800 square km of northern Ukraine, now represents the third-largest nature reserve in mainland Europe and has become an iconic — if accidental — experiment in rewilding.

From time to time some journalist and scientists do come to visit the place and it’s the tour guide’s responsibility to make sure that they do not spread radiation out of Chernobyl.

We need to learn from Chernobyl and decide how to coordinate international involvement in the investigation of a major disaster in a way that benefits both the most affected country and the world as a whole. That way we can reduce the risk of future disasters and improve our ability to deal with their consequences when they do occur.